This study will investigate the mechanism of regulation of human endothelial cell XO-XDH (xanthine dehydrogenase) by determining the effect of O2 tension on XO-XDH transcription, protein levels, activity, and molybdenum site integrity and; determine the specific contribution of XO to hypoxia-induced pro-inflammatory changes of human endothelial cells by creating XO-XDH under- and over-expressing HUVEC (human umbilical vein endothelial cells) transfectants containing the human XO- XDH gene in the antisense or sense orientations.